1. Field of the Invention
The present invention relates generally to measuring resistivity of media such as liquids, gases or other objects within a geologic formation through a well casing. Specifically, the present invention relates to through casing resistivity measurement in permanently installed downhole hydrocarbon production environments. The present invention provides an apparatus and method for measuring the resistivity of a formation in the vicinity of a well. The current invention, in one embodiment, is an apparatus that can be fixed inside the casing of an oil or gas producing well in many configurations for measuring the resistivity of the surrounding formation. The apparatus monitors changes in resistivity by continuous or periodic sampling. Any medium which provides for changes in resistivity are measurable, including for example changing concentrations of oil, water and gas in the formation. The apparatus can also detect the location and amplitude of said resistivity change in single or multiple directions, and at distances that will help operators of wells adjust their production management and their reservoir management activities. The invention includes fixed-inside-the-casing sensors, the associated method of introduction and operation in the well, the method of collecting the data and transmitting to the above ground control systems, and the interpretation and display of the results.
2. Description of Related Art
In the development and production of oil and gas reservoirs, there is a very significant need to increase the amount and accuracy of information regarding the composition and changes in the composition of the resource-bearing zones of the formation. Resistivity measurement has long been used to characterize properties of the immediately surrounding substrates prior to the inception of production. However, such measurements can only be taken prior to setting casing or while the formation itself is otherwise xe2x80x9cexposedxe2x80x9d to the logging tool i.e., without an interceding material that acts as a barrier between the logging tool and the formation substrate. Thus, there has always been a need to provide the capability for continuous or periodic measurements of formation resistivity for a permanent hydrocarbon production well installation. Specifically, there has been a need to be able to xe2x80x9csee throughxe2x80x9d the production tubing or well casing to the geologic formation or formation substrate located outside a section of the production well. There also has long been a need to provide resisitivity measurements without interruption of the hydrocarbon production well. There is also a great need for the ability to simultaneously (i) generate a magnetic flux, by conventional means, within the confines of a permanently installed hydrocarbon production well, (ii) create magnetic transparency zone within and through the well casing or production tubing of such a hydrocarbon production well, and (iii) receive and measure through the well casing or production tubing magnetic flux that may be generated in the media in the geologic formation outside of the well casing. It is already known to those skilled in the art that such measurements can provide information about the resistivity of media, and hence the composition or change in the media outside of the well.
However, existing measurements of the resistivity of the media within a geologic formation are taken with logging tools deployed prior to commencement of actual production or in a work-over situation. Yet, after production is underway, it is generally not possible to measure the resistivity of the surrounding geologic formation without interruption of the well production and penetration or removal of the well casing. As is known to those skilled in the industry, the formation resistivity varies as a result of (among other reasons) the changing proportion of hydrocarbon to water. Having the ability to measure at selected locations and directions over time would provide for the unique ability to monitor, for example, the change in the percentage of water versus either oil, gas, or other materials approaching the well, far in advance of such change in fluids actually entering the well. The benefits of such measurements include the ability to see changes in the resistivity of the formation over time in a non-obtrusive and highly accurate way even after the well is in production.
The invention subject of this invention provides a method and apparatus for continuous or periodic measurements of formation resistivity for a permanent hydrocarbon production well installation. Well casing and production tubing are commonly manufactured of materials that are electrically conductive and magnetically permeable. These materials are referred to as EM Barriers or Barrier Materials. The present invention creates transparencies within electrically conductive and magnetically permeable materials. Such materials include ferromagnetic and paramagnetic materials. Specifically, the method and apparatus of the invention creates transparencies in the well casing or production tubing. These transparencies permit magnetic flux to be transmitted through the casing, etc. The method and apparatus of the invention further includes the capability of generating magnetic flux to magnetically saturate a portion of the well casing or production tubing, transmitting magnetic flux through the transparency and measuring magnetic flux received from the surrounding geologic formation. The present invention also provides the ability to perform these activities continuously without interruption of the well production.